The present invention concerns a composition comprising an ethylene polymer and an ethylene copolymer, and its use for the manufacture of pipes. It also concerns a process of manufacture of that composition.
Patent application EP-A-0 603 935 describes a composition which comprises an ethylene polymer with high fluidity index (MI2 from 5 to 1000 g/10 min) and an ethylene polymer with low fluidity index (MI5 from 0.01 to 2 g/10 min) and is prepared in at least two reactors in series, the ratio by weight of these polymers being (30 to 70):(70 to 30). That patent application discloses more specifically a composition comprising an ethylene homopolymer having an MI2 of 168 g/10 min and an ethylene and butene copolymer having an MI5 of 0.21 g/10 min.
Patent application EP-A-0 897 934 describes a composition comprising an ethylene polymer with high fluidity index (MI2 from 5 to 1000 g/10 min) and an ethylene and hexene polymer with low fluidity index (MI5 from 0.01 to 2 g/10 min), the ratio by weight of these polymers being (30 to 70):(70 to 30) and the ratio of the MI2 of the ethylene polymer to the MI5 of the ethylene/hexene copolymer being less than 20000.
The compositions described in those patent applications present in general utilisation properties and mechanical properties which render them appropriate to be used for the manufacture of various formed objects, in particular for the manufacture of pipes for the conveying of fluids under pressure. The compositions explicitly described in those patent applications present mechanical properties, and more specifically a creep resistance, which make it possible to attribute to them a classification of MRS 8 or 10 according to standards ISO 9080 and ISO 12162. When these compositions are used for the manufacture of pipes with suitable dimensions, these pipes withstand certain pressures and certain temperatures such that extrapolation shows that the pipes have at 20° C. a resistance to circumferential stresses respectively of at least 8 and 10 MPa for at least 50 years. There is always a demand for compositions having a better creep resistance so as to be able to manufacture pipes withstanding a still greater circumferential stress while maintaining or improving the other mechanical and utilisation properties, in particular resistance to slow propagation of cracks (ESCR) and resistance to rapid propagation of cracks (RCP).
The object of the present invention is to provide ethylene polymer compositions having a better compromise between processing properties and mechanical properties than compositions obtained by known processes of the state of the art.